Question Thermal paste left dark stain on waterblock and I can't get it off with anything. Any ideas why?

[OG.BTW]

I was using a stock cooler from a 2600X but switched to a Celsius S36 after I got it. However, I didn't have any TP lying around and just kept whatever TP was left on the CPU because the preapplied compund on the S36 was very badly smudged. When I removed it a little later, there was a darkish stain left on the block that I couldn't remove. Any ideas why?

I am kinda new to PCs (5-ish months) so if this is a really stupid question sorry.

 

[USAFRet]

Switching coolers on the CPU, you really really really needed to clean off ALL the old stuff, before putting on the new one.
If the paste on the new block was smudged or otherwise not usable, you needed to clean all that off as well, and apply something new.

 

[mark1776]

Try isopropyl alcohol with cloth or paper towel to remove the old paste. Sometimes it takes a bit of effort. As for actual staining, I don't know what material the block is and what the paste was. I could just be residue left in the fine surface texture of the block, or it could be a chemical reaction.. Both nickel and copper will oxidize to a black color under some circumstances, but I haven't seen it with any thermal compound I've used.